It is desirable for plastic bags, particularly those used to contain bulk waste materials, to be resistant to damage by puncture and yielding under stress. Films with high strength characteristics, including ultimate tensile strength and puncture toughness, are needed in such applications. Additionally, thin films that exhibit high strength requirements provide a better cost-performance relationship for the consumer. Currently, such bags are most commonly produced from polyolefin films, including polyethylene films.
For many years, high performance polyolefins, such as low density polyethylene (LDPE), have been readily available at a low manufacturing cost sufficient to justify commercial use in trash bags, including heavy duty garbage bags, leaf bags and trash can liners. The use of polyethylene, more particularly low density polyethylene, allows for the production of bags with remarkably thin gauge and flexibility while maintaining high strength characteristics such as puncture toughness and ultimate tensile strength.
More recently, linear low density polyethylene (LLDPE) has been used in place of conventional highly branched LDPE in many film applications, including bags. LLDPE is widely recognized as being tougher and stronger than LDPE, thus contributing to reduced bag failures, including punctures and splitting under stress. In particular, LLDPEs made with metallocene or single site catalysts, and LLDPEs containing hexene and/or octene comonomers, including “super hexenes,” have been used to provide improved toughness.
Prior art polyethylene film bags are generally manufactured by blown film processes and are frequently characterized by limited toughness and strength properties
U.S. Patent Publication No. 2004/0077810 to Marechal discloses a linear low density polyethylene resin produced with a metallocene or late transition metal catalyst having a density of from 0.906 to 0.940 g/cm3, a Melt Index of from 0.01 to 150 g/min., and a molecular weight distribution of less than 4. The publication further discloses blown films. Marechal does not disclose a cavitated, post-quench biaxially oriented film.
U.S. Pat. No. 6,696,166 to Bayley et al. (Nova Chemicals International S.A.) discloses a plastic film suitable for the preparation of packaging and trash bags having a desirable balance of physical properties. The film is prepared using a pseudohomogeneous linear low density polyethylene resin on a blown film extrusion line using specific extrusion conditions and a specific annular die. A Ziegler-Natta catalyst is employed in the production of the resin. Bayley et al. do not disclose a cavitated, post-quench biaxially oriented film.
U.S. Pat. No. 6,689,857 to Larter et al. (ExxonMobil Oil Corporation) discloses a high density polyethylene film having high-biaxial orientation and optional cavitation for special film applications. Specifically, the film has an orientation imbalance with orientation in the machine direction ranging from 5:1 to 8:1 and transverse orientation ranging from 6:1 to 15:1. The film comprises 50% to 90% high density polyethylene. Larter et al. do not disclose use of a linear low density polyethylene.
U.S. Pat. No. 6,391,411 to Duckwall, Jr. et al. (Printpack Illinois, Inc.) discloses an oriented, high molecular weight, high density polyethylene film with superior toughness properties, a method for making the film, and articles made therefrom, particularly bags. Duckwall, Jr. et al. do not teach a cavitated and biaxially oriented linear low density polyethylene.
U.S. Pat. No. 4,511,609 to Craver et al. (Union Carbide Corporation) discloses a multilayer polyethylene film suitable for use in the manufacture of garbage and trash bags wherein the film comprises a first outer layer of low pressure, low density polyethylene, a core layer of high pressure, low density polyethylene and a second outer layer of high pressure, low density polyethylene. Craver et al. do not disclose a cavitated film or biaxial orientation.
U.S. Pat. No. 6,228,505 to Agent et al. (Mobil Oil Corporation) discloses a biaxially oriented polyethylene film having unidirectional tear characteristics in the machine direction and the method of making same. The film comprises a core layer of a medium density polyethylene, an optional cavitating agent in the core layer, and at least one skin layer also made from polyethylene. Contrary to the teachings of Agent et al., the present invention is directed to a film that withstands tears.
None of the prior art films described above combine desired puncture toughness and ultimate tensile strengths with low manufacturing costs and excellent optical and tactile properties. Opportunities exist for cavitated, biaxially oriented polyethylene films to replace current blown films for use in bag applications, including garbage bags, requiring high puncture toughness and ultimate tensile strength properties. Clear polyethylene films and polyethylene films containing colorants and/or pigments according to this invention are also contemplated where those films are biaxially oriented and possess the desired puncture toughness and ultimate tensile strength. The present invention meets these and other needs.